The invention relates generally to the field of polymeric optical articles. More particularly, the invention concerns polymeric optical materials and small articles, such as plastic microlenses, that maintain stable performance characteristics over a broad temperature range.
Plastic microlenses and glass microlenses often perform the same function in optical systems, such as in cameras, microscopes, and fiber optic devices. The two main attributes that separate plastic lenses from glass lenses are cost and optical stability. Plastic microlenses typically cost {fraction (1/10)}th to {fraction (1/100)}th the price of a similar glass microlens. On the other hand, the stability of the refractive index of a glass microlens with respect to temperature and humidity is typically 100 times better than that of a plastic microlens. Other reasons why a plastic microlens might be used over a glass microlens include: lower weight and higher impact resistance.
Microlenses represent a special case in the manufacturing of optical articles due to their very small size: from approximately 1 millimeter in diameter and often as small as 10 microns in diameter. While some microlenses can be manufactured by molding, in the case of extremely small microlenses, photolithographic techniques are often used. In photolithography, the difference in cost between plastic and glass microlenses is due largely to the difference in manufacturing processes that are required for the two materials. Glass microlenses and plastic microlenses for spherical lenses and aspheric lenses can be made using gray scale lithography. Stepped optics and some diffractives can also be made in glass and plastic using half tone lithography. In addition, plastic microlenses can be made using modulated laser writing for spherical lenses and aspheric lenses as well as ink jet or printed lithography for spherical lenses by a reflow technique. In all of these exposure techniques, both laser beam and UV radiation sources can be used. Of all the manufacturing techniques, gray scale lithography is the most expensive since it requires the use of a very expensive gray scale photomask ($30,000 typical) along with an iterative definition of the exposure process to get the desired surface profiles or optical curves. The other manufacturing techniques available for making plastic microlenses are all much less expensive because the gray scale mask is not required so that the overall production costs are much less for plastic microlenses made from photoresist than glass microlenses.
In contrast, the difference in optical stability between plastic and glass is due to differences in their basic material properties. This difference in optical stability results in substantially more variation in focus and light loss in articles such as fiber optic devices when plastic microlenses are used in place of glass microlenses. What is desired, and is a remaining challenge in the art, is a material with the optical stability of glass that processes like a plastic. While some optical plastic materials such as cyclic olefins greatly improve the refractive index stability with respect to humidity, improving the refractive index stability of plastics with respect to temperature has remained an opportunity.
A study on the competing fundamental material characteristics that determine the sign and the magnitude of the dn/dT of glasses is available, for instance, by Lucien Prod""homme, xe2x80x9cA new approach to the thermal change in the refractive index of glasses,xe2x80x9d Physics and Chemistry of Glasses, Vol. 1, No. 4, August 1960. The two competing effects that determine the dn/dT in glasses are the density change which produces a negative dn/dT and the electronic polarizability which produces a positive dn/dT. The net dn/dT in a glass material depends on which effect dominates. In optical plastics however, there is not an electronic polarizability so that all unfilled plastic materials have negative dn/dT values. None the less, the article by Prod""homme does identify the possibility of using glass-like fillers with positive dn/dT values to substantially alter the dn/dT of a filled plastic composite material.
Nanoparticulate fillers have been used to modify the index of refraction of optical plastics. By using a filler small enough that it is well below the wavelength of visible light (400-700 nm), the filler will not scatter the light and the filled plastic can retain its transparency. WIPO Patent WO97/10527 describes the use of nanoparticles to increase the refractive index of plastics for opthalmic applications. In addition, technical references that describe the addition of nanoparticles to increase the refractive index of plastics include: C. Becker, P Mueller, H. Schmidt; xe2x80x9cOptical and Thermomechanical Investigations on Thermoplastic Nanocomposites with Surface-Modified Silica Nanoparticles,xe2x80x9d SPIE Proceedings Vol. 3469, pp. 88-98, July 1998; and, B. Braune, P. Mueller, H. Schmidt; xe2x80x9cTantalum Oxide Nanomers for Optical Applications,xe2x80x9d SPIE Proceedings Vol 3469, pp. 124-132, July 1998. While these references disclose the use of nanoparticles to modify refractive index of optical plastics they do not discuss the issue of refractive index stability with respect to temperature which requires a different set of characteristics in the nanoparticle.
U.S. Pat. No. 6,020,419 issued to M. Bock, et al., discloses the use of nanoparticulate fillers in a resin based coating for improved scratch resistance. U.S. Pat. No. 5,726,247 issued to M. Michalczyk, et al., also describes a protective coating that incorporates inorganic nanoparticles into a fluoropolymer. While scratch resistance is important in plastic optics, the nanoparticles that would be suitable for scratch resistance would be very different from those with the specific properties needed to improve refractive index stability with respect to temperature.
U.S. Pat. No. 3,915,924 issued to J. H. Wright describes a nanoparticulate filled clear material for filling voids. U.S. Pat. No. 5,910,522 issued to H. Schmidt, et al., describes an adhesive for optical elements that includes nanoscale inorganic particles to reduce thermal expansion and improved structural properties at elevated temperatures. While the inventions described in these patents represents some progress in the art, none of them address specific optical properties of the modified plastic material particularly as these properties relate to temperature sensitivity.
WIPO Patent WO9961383A1 discloses a method for producing multilayered optical systems that uses at least one layer that contains nanoparticulate fillers to form a layer with a different refractive index than the substrate to create an interference filter or an antireflection layer. Obviously, this patent is addressing another form of modification of the index of refraction and such is not concerned with the stability of the index of refraction with respect to temperature.
Skilled artisans will appreciate that a wide variety of materials are available in nanometer particle sizes that are well below the wavelength of visible light. Representative materials may be acquired from companies such as Nanophase Technologies Corporation (1319 Marquette Drive, Romeoville, Ill. 60466) and Nanomaterials Research Corporation (2620 Trade Center Avenue, Longmont, Colo. 80503). By selecting nanoparticle materials based on properties other than index of refraction, our experience indicates that it is now possible to modify other optical properties of plastics.
While there have been several attempts to modify properties of plastics using nanoparticles, none of these attempts have proven successful in producing optical plastic articles with temperature stable optical properties while retaining important processing characteristics.
Therefore, a need persists in the art for optical plastic articles, such as microlenses, and a method of making same that have temperature stable optical properties.
It is, therefore, an object of the invention to provide an optical nanocomposite material that has reduced temperature sensitivity.
Another object of the invention is to provide an optical article, such as a plastic microlens, that maintains stability over a broad range of temperatures.
Yet another object of the invention is to provide a method of manufacturing an optical article having reduced temperature sensitivity.
It is a feature of the optical article of the invention that a select nanoparticulate is dispersed into a plastic photoresist host material having a temperature sensitive optical vector that is directionally opposed to the temperature sensitive optical vector of the nanoparticulate filler.
To accomplish these and other objects, features and advantages of the invention, there is provided, in one aspect of the invention, a photoresist nanocomposite optical plastic article comprising: a photoresist host material having a temperature sensitive optical vector x1 and substantially transparent inorganic nanoparticles having a size less than 40 nm dispersed in the photoresist host material having a temperature sensitive optical vector x2, wherein the temperature sensitive optical vector x1 is directionally opposed to temperature sensitive optical vector x2 
In another aspect of the invention, there is provided a method of manufacturing a photoresist nanocomposite optical plastic article, comprising the steps of:
(a) providing a photoresist host material having a temperature sensitive optical vector x1 and substantially transparent inorganic nanoparticles having a temperature sensitive optical vector x2, wherein temperature sensitive optical vector x1 is directionally opposed to temperature sensitive optical vector x2;
(b) dispersing the substantially transparent inorganic nanoparticles into the photoresist host material forming a photoresist nanocomposite material; and,
(c) forming the photoresist nanocomposite material into the photoresist optical plastic article.
Hence, the present invention has numerous advantageous effects over existing developments, including: (1) the resulting nanocomposite has a significantly lower dn/dT (change in refractive index with temperature); (2) microlenses made with the nanocomposite material have more stable focal length over a given temperature range; (3) low levels of dn/dT are achievable in the nanocomposite material with reduced loading of the nanoparticulate; (4) the viscosity of the nanocomposite material is not significantly higher than the base plastic so that conventional plastic processing techniques can be used; and, (5) the nanocomposite material has improved barrier properties so that the change of refractive index with respect to humidity will be reduced compared to the base plastic.